Uncaught ReferenceError: ExtPay is not defined on popup.js

[Johnek1989]

When I am trying to use ExtPay function outside background.js I getting errors like above. I found:
// background.js

importScripts('ExtPay.js') // or import / require if using a bundler

var extpay = ExtPay('sample-extension'); // Careful! See note below
extpay.startBackground();
Note about service workers: In the example above extpay will become undefined when accessed in service worker callbacks. To use extpay in service worker callbacks, redeclare it like so:

chrome.storage.local.get('foo', function() {
var extpay = ExtPay('sample-extension');
// ...
})

but i have no idea how to implement bottom part of code.

[Johnek1989]

popup.js is workig I forgot to add "ExtPay.js" to popup.html

Now i have same issue on content script:

const extpay = ExtPay('agenthelpertest');

extpay.getUser().then((user) => {
if (user.paid) {

[Johnek1989]

Solved again the issue was that is manifest.json file I mess up order: "js": [ "content.js","ExtPay.js"] insdeat of "js": ["ExtPay.js", "content.js"],